# 函数返回字典
def build_person(first_name, last_name, age=None): # None表示变量没有值
    person = {
        'first': first_name,
        'last': last_name,
        }
    if age:
        person['age'] = age
    return person

print(build_person('w', 'z'))
print(build_person('w', 'z', 12))
